The author presents the concept of “collusive infidelity” and the role of projective identification as ubiquitous in the unconscious encouragement of infidelity through triangulation. He also discusses how to work with this dynamic in couples therapy, particularly by attending to the clinician's own countertransference reactions. To illustrate these ideas he provides a commentary on a session where collusion dynamics were observed. Finally, he examines how the concept of collusive infidelity can provide a link between psychoanalytic and family systems theories and suggests that the concept of collusive infidelity can be helpful when working with a couple who are in the wake of an affair.